This invention relates generally to rotation sensors, and, more particularly to a fiber optic rotation sensing interferometer.
In the fields encompassing navigation, guidance and geophysical measurement, conventional gyroscopes leave much to be desired in their sensitivity in the measurement of rotation rates. In recent years, however, a dramatic development has taken place in optical technology with the invention of the so called laser gyroscope. This device combines the properties of the optical oscillator, the laser and an optical effect known as the Sagnac effect to produce an integrating rate gyroscope. A good description of a laser gyroscope can be found in U.S. Pat. No. 4,013,365.
As pointed out in U.S. Pat. No. 4,013,365, the conventional laser gyroscope is a laser that has three or more reflectors arranged to enclose an area. The three reflectors, preferably mirrors, together with the light-amplifying material in the laser path, comprise an oscillator. In fact, there are two oscillations, one that has energy traveling clockwise and one that has energy traveling counterclockwise around the same physical cavity. The two frequencies at which the oscillator operates are determined by the apparent or effective optical path length of the cavity in the clockwise and counterclockwise directions. In order to sustain oscillation, two conditions must be met: (1) the gain must be at least equal to unity at some power level set by the amplifying medium, and (2) the number of wavelengths in the cavity must be an exact integer. If the first condition is to be achieved, the laser operation must be such that the amplifying medium has sufficient gain to overcome the losses at the reflectors and the other elements in the laser path. In addition, the wavelength must be an exact submultiple of the path around the cavity. This last condition actually determines the oscillation frequency of the laser.
When the enclosed ring is rotated in inertial space the clockwise and counterclockwise paths have different apparent lengths. The path difference in these two directions causes the two oscillators to operate at different frequencies. The difference is proportional to the rate at which the ring is rotating since path difference is proportional to inertial rotation rate. The readout of the gyroscope is accomplished by monitoring the frequency difference between the two oscillators. For example a device which falls within the laser gyroscope field is the ring laser gyroscope of the type described by Post, E. J. Rev. Mod. Phys. 39, 2, 475 (1967).
Unfortunately, the current devices using oscillators as described, will not provide an output signal at very low rotation rates as required in such applications as navigation, guidance, etc., due to the fact that the two oscillations, corresponding to propagation of signals in opposite senses around the loop tend to interact in such a way that they lock and give only one frequency and therefore no signal. This mode locking has been a fundamental deficiency to the use of a laser oscillator for rotation sensing.